1. Field of the Invention
The present invention relates to traffic control and registration by means of vehicle identification.
2. Background of the Related Art
A conventional approach to traffic control is to use an automated vehicle identification system, comprising electronic identification devices mounted on vehicles and electronic reading and writing devices, both equipped with an induction transmitter and receiver. Electronic identification devices also include: at least one HF-band transmitter with range of 10-90 cm, at least one UHF-band transmitter with range of 300-1000 cm, and an electronic memory unit (see US20120056725).
This system has a number of disadvantages:
1. It is overly complex, since it requires using both HF and UHF bands to connect electronic identification devices mounted on vehicles and electronic reading devices.
2. UHF-band antennas can't be placed under the roadway because of high wave attenuation. They can't be placed on the roadway as well, because certain weather conditions, such as rain, mud, snow and ice, decrease their range and reliability by increasing wave attenuation and distorting the orientation diagram of their electromagnetic fields. A UHF-band RFID-system comprising a transponder, an antenna and a reader is operable only under conditions of radio visibility, while large objects, such as vehicles or houses, whose size is proportionate to the wavelength, can cause a complete loss of signal, as they shield electromagnetic waves, or can cause interference leading to signal distortion.
To enable the system to identify vehicles, it is necessary to construct posts or farms over the road, where reader antennas with transponders can be mounted. However, such constructions do not solve the problem of interference for UHF band, which leads to connection failures and decreases reliability of the system. Interference also makes it difficult to locate the vehicle on the road. Moreover, any construction over the road or at the roadside affects the architectural look of the site.
3. HF RFID-readers equipped with antennas, such as long-range induction systems, can read information from standard transponders at the distance of 80-90 cm. But the size of such antennas should be at least ten times less than the wavelength of the induction system to get focused parameters. Thus, the typical range of a standard frame antenna forming a magnetic field with intensity of at least 150 mA/m for standard transponders, can't exceed the box with dimensions of 800×600×800 mm. The perimeter of such antenna should be equal to ⅛ of the wavelength (see, e.g., HF Long Range reader ID ISC.LR(M)2500 by FEIG Electronic). Such range is insufficient for identification of fast-moving objects, even those equipped with transponders, because the time required to read identification data exceeds the time a vehicle with transponder needs to pass through the antenna range. Even if positioned across the road, this antenna won't still be able to cover the entire lane.
4. Such systems have a very short range, and the sizes of vehicles can vary within meters, thus if the antenna is placed over the road or at the roadside, it can't provide reliable vehicle detection. Moreover, any constructions over the road or at the roadside affect the architectural look of the site and compromise traffic safety.
5. If an HF-band antenna is placed under the roadway, i.e., in a conductive environment (grounded conductor), it is exposed to stray capacitance and stray currents, which distort the frame, decreasing its Q-factor and shifting its resonant frequency. It is thus impossible to tune the system properly, which is why it can't function as needed.
These disadvantages thus limit the application of this solution.
Another conventional approach comprises a license plate implemented as a dielectric plate (see U.S. Pat. No. 5,621,571). Its uppermost layer is reflective and covered with a combination of letters and numbers for visual recognition. The underlying dielectric layer has an antenna, which is capable of transmitting and receiving a 915 MHz frequency signal and is equipped with a connecting feeder.
This approach can also be used for radio-frequency identification of a vehicle.
The problem of this approach is that it is impossible to make the license plate autonomous, since its antenna requires an extra connection. Such connections pose difficulties when mounting the plate, decrease its reliability, lead to signal losses, and make it impossible to operate in a passive mode.
Another problem is the need to excite the transmitter, which is already rather powerful, since a separate antenna unit requires high-frequency operation (in this case, 915 MHz). At such frequencies, there is high signal attenuation, and thus it is difficult, or even impossible, for the license plate to operate in passive mode in radio-frequency communication.
Moreover, if a high carrier frequency is used, its wavelength can be proportionate to the size of the license plate, and thus it is prone to negative effects of multiple reflections and shadowing caused by nearby objects (e.g., other vehicles). Multiple reflections distort the antenna orientation diagram, which makes it difficult, or even impossible, to detect and locate the plate, thus decreasing its reliability when interacting with vehicle identification systems.
These disadvantages thus limit the application of the device.
Another conventional device is a sensor implemented as an induction frame and placed under the roadway (see U.S. Pat. No. 3,651,452). When a vehicle enters its range, its inductive impedance is disturbed. The other constituent parts of the known device are: a fixed frequency generator, matching devices, which are able to generate a monitoring signal if there is a vehicle nearby, and devices reacting on that signal.
This device is able to register a vehicle moving past the induction frame.
However, the conventional device has the following disadvantages:
1) it can't identify vehicles, since it doesn't comprise any means of signal decoding;
2) its reliability is low, as it can register two vehicles moving closely to each other as one vehicle, or there can be errors in registering oversized vehicles;
3) there is a high chance for the device to miss a small-sized vehicle;
4) the device can malfunction because of a sudden change in weather conditions;
5) the induction frame is in no way protected from mechanical impact of moving vehicles; it is thus prone to breakdowns and has a low durability.
These disadvantages thus limit reliability and application of the device.